Boron reagents have long been known to have many applications in organic chemistry. Hydroboration is one of the most widely used organometallic reactions in organic synthesis and a wide variety of borane reagents have been produced which are tailored to many specific applications, as disclosed, for example, in K. Smith and A. Pelter, Comprehensive Organic Synthesis, Vol 8, (eds: B. M. Trost and I. Fleming) Pergamon Press, New York, 1991, p. 703. Hydroboration reactions are particularly useful for the conversion of alkenes and alkynes to alcohols or ketones. Generally, hydroboration simply involves the combination of an alkyne or an alkene (olefin) with a borane reagent in an appropriate solvent. One of the best known hydroborating reagents is 9-BBN (9-borabicyclo[3.3.1]-nonane) which is commercially available as a solid or as a solution in various solvents. A drawback to hydroboration of organics by reaction with 9-BBN is that they typically take several hours to go to completion.
Boron reagents have also been used as co-catalysts in catalyst systems used in the preparation of polymers such as polyethylene, a compound of major commercial importance, see X. Yang, C. L. Stern and T. J. Marks, J. Am. Chem. Soc., Vol 116, p. 10015, 1994. Polyethylene is a very versatile plastic used in a wide variety of industrially useful products including plastic containers, toys and saran wrap, to mention just a few. The physical properties of this plastic are highly dependent on the chemical structure of the polymer comprising polyethylene, which in turn is strongly influenced by the chemical structure of the homogeneous catalysts used in the polymerization catalysts with the olefin monomers which are reacted to form the polymer chains.
Typically, the polymerization catalyst systems are comprised of two components including a catalyst precursor and a co-catalyst which, when combined, form the species responsible for joining the olefin monomers into the long chain polymers. One of the components is a group 4-based metallocene chloride or alkyl complex, and the other is a Lewis acid co-catalyst which serves to activate the metallocene to form the active species. Specifically, combinations of group 4-based bent metallocene compounds with group 13 Lewis acids often results in catalysts capable of polymerizing olefinic substrates by ring opening metathesis, as disclosed in Grubbs, R. H. and Tumas, W., Science, Vol. 243, p.907, 1989; or by Ziegler-Natta type mechanisms as disclosed in Sinn, H. and Kaminsky, W., Adv. Organomet Chem, Vol. 18, p. 99, 1992.
The architecture and chemical structure of the supporting ligand for the group 4 metal influences rates of initiation, propagation and termination, as well as the regio and steriochemistry of propagation when .alpha.-olefins are polymerized. Consequently, the steric and electronic properties of the ligand framework are crucial in determining the properties of the product of the polymerization reaction. As a result, the design of new polymerization catalysts is a very active field with the goal being to produce polymerization catalysts for producing plastics with superior properties.
Therefore there has been a need for a single polymerization catalyst which incorporates the catalyst precursor and the co-catalyst into a single molecule. While this goal of combining the components of the polymerization catalyst into a single molecule has been pursued, to date such attempts have been unsuccessful due to the synthesis problems involved in producing such a molecular species. The most significant challenge arises because of the need to incorporate the co-catalyst moiety at the latest possible stage in the catalyst synthesis. Hydroboration offers a synthetic solution to this challenge, but prior to the present invention suitable hydroboration reagents have not been known.
Combining all of the features required in an ethylene polymerization catalyst system into a single molecule would be advantageous because the catalyst should be "self-activating" and form a zwitterionic active species and be more soluble.